The present document incorporates by reference the entire contents of Japanese priority document, 2003-038716 filed in Japan on Feb. 17, 2003.
1) Field of the Invention
The present invention relates to an optical scanner and an image forming apparatus.
2) Description of the Related Art
Optical scanners are widely used as a device for writing an image in the image forming apparatuses such as digital copying machines, optical printers, or facsimile machine. In the various types of image forming apparatus that use the optical scanner, the colorization is improved and accompanying this, the optical scanners are required to have high performance.
The characteristics of the optical scanner include constant velocity characteristic in optical scanning, spot diameter of a light spot, stability of the spot diameter, linearity of the scan line, optical scanning speed. It is ideal that all these characteristics are excellent, but it is difficult to achieve all superior characteristics. Moreover, some characteristics may deteriorate when other characteristics are excellent.
To achieve better optical characteristics, for example, to achieve better field curvature and a small spot diameter, it is now essential to adopt a xe2x80x9cspecial surface represented by an aspheric surfacexe2x80x9d for lenses in the optical system. A resin lens has been widely used in the optical system because it can easily achieve a lens having such a special surface at a low cost.
When the resin lens is formed by resin molding, there are problems such as occurrence of refractive index distribution at the time of cooling the lens in a mold, deformation such as xe2x80x9cshrinkagexe2x80x9d, and deterioration in the transfer accuracy on the molded surface.
Such problems are likely to occur when the molded resin lens is thick, and when a difference in thickness at a central portion and the peripheral portion of the lens is large. On the other hand, a certain degree of thickness difference between the central portion and the peripheral portion of the lens is required, in order to favorably correct the xe2x80x9cconstant velocity characteristicxe2x80x9d, which is important as the optical characteristics.
Recently, four-drum tandem type color image forming apparatuses have been put into practical use. In these four-drum tandem type color image forming apparatuses, four photosensitive drums are arranged in a feeding direction of transfer paper, these are optically scanned simultaneously to form an electrostatic latent image thereon, the electrostatic latent images formed are visualized by toners such as yellow, magenta, cyan, and black, and the respective color toner images are transferred onto the same transfer paper and superposed on each other, to thereby obtain a color image.
In the four-drum tandem type color image forming apparatus, generally a deflection unit is shared by all the optical beams that optically scan four photosensitive materials to achieve compactness. As a method of sharing such a deflection unit by the whole optical beams a counter-scanning method (Japanese Patent Application Laid-open No. H9-54263) and a one-side scanning method (Japanese Patent Application Laid-open No. 2001-4948, Japanese Patent Application Laid-open No. 2001-10107, respectively) are known. In the counter-scanning method, optical beams are made to enter from opposite sides of the deflection unit, and the optical beams are divided to the opposite sides of the deflection unit to perform deflection scanning. In the one-side scanning method in which the whole optical beams are made to enter from one side of the deflection unit, and the whole optical beams are directed in the same direction on one side of the deflection unit, to perform deflection scanning.
In such counter-scanning type and one-side scanning type color image forming apparatus, if a resin lens is used in the optical scanning optical system, optical characteristics of the optical scanning optical system involved in the optical scanning of the respective photosensitive materials become different, due to non-uniform temperature distribution in the apparatus resulting from the heat generated in the deflection unit. For example, there is a problem in that a phenomenon referred to as xe2x80x9cout-of-color registrationxe2x80x9d occurs in the color image formed by superposing toner images of the respective colors, due to uneven constant velocity characteristic, to thereby deteriorate the image quality. Further, when a plurality of color images are continuously formed, there is a problem in that occurrence of out-of-color registration changes with lapse of time, due to a temperature rise in the apparatus resulting from the continuous operation of the apparatus, and the hue in the formed color image changes, and hence the hue of the formed color images is different.
Japanese Patent Application Laid-open No. 2002-036625 describes a technique to correct such the out-of-color registration.
It is an object of the present invention to solve at least the problems in the conventional technology.
An optical scanner according to one aspect of the present invention includes a deflecting unit that deflects optical beams emitted from a light source at a constant angular velocity; and a scanning imaging lens that condenses the optical beams deflected on a surface to be scanned, the scanning imaging lens including at a first lens that is arranged closer to the deflection unit and a second lens that is arranged closer to the surface to be scanned. The first lens is formed by resin molding, and has a positive refracting power in the horizontal scanning direction, with zero or close to zero refracting power in the vertical scanning direction, and has a function of correcting a constant velocity characteristic in the optical scanning. The second lens has a weak refracting power in the horizontal scanning direction and a strong positive refracting power in the vertical scanning direction. When it is assumed that a deflection angle of optical beams deflected by the deflection unit is xcex8, an image height at the deflection angle xcex8 of the light spot is H(xcex8), and an ideal image height at the deflection angle xcex8 of the light spot is kxcex8, designating k as a constant, the linearity Lin of the scanning imaging lens defined by Lin=|[{(dH(xcex8)/dxcex8)/k}xe2x88x921]|xc3x97100 (%) satisfies a condition 2.0 less than Lin less than 10.0(%).
An image forming apparatus according to another aspect of the present invention includes the optical scanner according to the above aspect.
The other objects, features, and advantages of the present invention are specifically set forth in or will become apparent from the following detailed descriptions of the invention when read in conjunction with the accompanying drawings.